Noise control constitutes a rapidly growing economic and public policy concern for the construction industry. Areas with high acoustical isolation (commonly referred to as ‘soundproofed’) are requested and required for a variety of purposes. Apartments, condominiums, hotels, schools and hospitals all require rooms with walls, ceilings and floors that reduce the transmission of sound thereby minimizing, or eliminating, the disturbance to people in adjacent rooms. Soundproofing is particularly important in buildings adjacent to public transportation, such as highways, airports and railroad lines. Additionally theaters, home theaters, music practice rooms, recording studios and the like require increased noise abatement. Likewise, hospitals and general healthcare facilities have begun to recognize acoustical comfort as an important part of a patient's recovery time. One measure of the severity of multi-party residential and commercial noise control issues is the widespread emergence of model building codes and design guidelines that specify minimum Sound Transmission Class (STC) ratings for specific wall structures within a building. Another measure is the broad emergence of litigation between homeowners and builders over the issue of unacceptable noise levels. To the detriment of the U.S. economy, both problems have resulted in major builders refusing to build homes, condos and apartments in certain municipalities; and in widespread cancellation of liability insurance for builders. The International Code Council has established that the minimum sound isolation between multiple tenant dwellings or between dwellings and corridors is a lab certified STC 50. Regional codes or builder specifications for these walls are often STC 60 or more. It is obvious that the problem is compounded when a single wall or structure is value engineered to minimize the material and labor involved during construction.
It is helpful to understand how STC is calculated in order to improve the performance of building partitions. STC is a single-number rating that acts as a weighted average of the noise attenuation (also termed transmission loss) of a partition across many acoustical frequencies. The STC is derived by fitting a reference rating curve to the sound transmission loss (TL) values measured for the 16 contiguous one-third octave frequency bands with nominal mid-band frequencies of 125 Hertz (Hz) to 4000 Hertz inclusive, by a standard method. The reference rating curve is fitted to the 16 measured TL values such that the sum of deficiencies (TL values less than the reference rating curve), does not exceed 32 decibels, and no single deficiency is greater than 8 decibels. The STC value is the numerical value of the reference contour at 500 Hz. For maximum STC rating, it is desirable for the performance of a partition to match the shape of the reference curve and minimize the total number of deficiencies.
An example of materials poorly designed for performance according to an STC-based evaluation is evident in the case of many typical wood framed wall assemblies. A single stud wall assembly with a single layer of type X gypsum wallboard on each side is recognized as having inadequate acoustical performance. That single stud wall has been laboratory tested to an STC 34—well below the STC 50 building code requirement. A similar wall configuration consisting of two layers of type X gypsum wall board on one side and a single layer of type X gypsum board on the other is an STC 36—only a slightly better result.
Various construction techniques and products have emerged to address the problem of noise control, such as: replacement of wooden framing studs with light gauge steel studs; alternative framing techniques such as staggered-stud and double-stud construction; additional gypsum drywall layers; the addition of resilient channels to offset and isolate drywall panels from framing studs; the addition of mass-loaded vinyl barriers; cellulose-based sound board; and the use of cellulose and fiberglass batt insulation in walls not requiring thermal control. All of these changes help reduce the noise transmission but not to such an extent that certain disturbing noises (e.g., those with significant low frequency content or high sound pressure levels) in a given room are prevented from being transmitted to a room designed for privacy or comfort. The noise may come from rooms above or below the occupied space, or from an outdoor noise source. In fact, several of the above named methods only offer a three to ten decibel improvement in acoustical performance over that of standard construction techniques that give no regard to acoustical isolation. Such a small improvement represents from a just noticeable difference to an incremental improvement, but not a soundproofing solution. A second concern with the above named techniques is that each involves the burden of either additional (sometimes costly) construction materials or extra labor expense due to complicated designs and additional assembly steps.
More recently, an alternative building noise control product having laminated panels utilizing a viscoelastic glue has been introduced to the market. Such panels are disclosed and claimed in U.S. Pat. No. 7,181,891 issued Feb. 27, 2007 to the assignee of the present application. This patent is hereby incorporated by reference herein in its entirety. The laminated panels disclosed and claimed in the '891 Patent include gypsum board layers assembled into a single soundproof assembly. The advantage of these laminated panels is that they eliminate the need for additional materials such as resilient channels, mass loaded vinyl barriers, and additional layers of drywall during initial construction. The resulting panel improves acoustical performance over the prior art panels by ten or more decibels in most cases and does so without the need for additional layers of construction materials or costly additional labor. However, the described panels are, in general, only optimized with regard to acoustics, with little regard to other material attributes, such as structural integrity. In all of these panels, one or more viscoelastic adhesives (for damping) are incorporated into the laminated panel solely for the purposes of damping and panel adhesion. As will be demonstrated below, such adhesive is designed to damp sound energy but may exhibit poorer performance with regard to panel shear resistance, creep of layers across each other, or the integrity of laminate after cutting the panel into smaller subpanel elements. The term subpanel refers to the fact that, often times, panels are cut to a fraction of the whole panel dimension to accommodate the dimensions and details of real rooms. For this reason, these prior art damped panels compromise the laminates' integrity in terms of the best acoustic energy isolation across its entire face.
A figure of merit for gauging the sound attenuating qualities of a material or method of construction is the material's Sound Transmission Class (STC). STC is a rating which is used to rate partitions, doors and windows for their effectiveness in reducing the transmission of sound. The STC rating is a result of acoustical testing, being derived from a best fit to a set of curves that define the sound transmission class. The test is conducted in such a way that the resulting measurement of the partition is independent of the test environment. The STC is therefore a number for the partition performance only. The STC measurement method is defined by ASTM E90 “Standard Test Method Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements,” and ASTM E413 “Classification for Sound Insulation,” used to calculate STC ratings from the sound transmission loss data for a given structure. These standards are available on the Internet at http://www.astm.org.
A second figure of merit is the panel's structural integrity. For some embodiments of the present invention, one or more test methods may be employed to determine the structural integrity of the damped panels. Such tests are designed to investigate the use of a combination of a structural adhesive and a visco-elastic adhesive to improve the stability of damped panels. The test methods may include the following separate tests performed on the structures: an adhesive bond strength test, a stack holding strength test, a creep resistance test, and a shear strength test. In a stack holding strength test, the materials are formed under normal manufacturing conditions and stacked under weight, for drying. The specimens to be tested are supported by one another and by a number of gypsum risers (4 in some embodiments of the test method) at the base of the stack.
The stack holding strength test may include the arrangement of a large number of panels stacked in a lift. This stack may hold for example thirty to fifty panels, or more. The stack is then placed without end-caps or strapping.
The additional creep resistance test may include one board taken from the damped panel stack and leaned against a wall at a 20 degree angle. The board is left to sit under its own weight and the movement of the unrestrained top half of the board is monitored and recorded at regular intervals (e.g. twelve hours apart), until an offset between the two halves of the panels reaches one quarter inch. The time required to reach such offset is recorded.
Typically, a board left overnight (i.e. for over 12 hours) that shows no sign of an offset is qualified under this test protocol.
For the shear strength test, several panels are glued together in a stack and allowed to dry. In some cases, the panels in the stack are about 10 ft long. The stack is then lifted and lowered with a forklift, ten times. The forks on the forklift are set closely together (e.g. at six inches apart from each other). This test subjects the adhesive bond to bending stresses higher than those that would normally be encountered during typical panel transport and bulk handling. The conditions of the boards are monitored while the boards are elevated. If the ends of the boards are offset, then the adhesive has failed, which allowed the panel halves to slide across each other. For the purposes of evaluation, the board is then split and the adhesive bond is viewed to see if the failure occurred in the substrate or the adhesive.
A final evaluation method is the adhesive bond strength test. The adhesive bond strength test tests the bond of the adhesive to the substrate, as compared to the internal structural strength of the substrate. In some embodiments, a wooden dowel or handle is glued to a paper faced gypsum board and bare gypsum substrate, using the adhesive under test. After curing the adhesive, the wooden dowel or handle is torn from the substrate by pure tension; the resulting two pieces are analyzed to see if some of the substrate was pulled off by the adhesive. The amount of adhesive failure is measured by the ratio of the area of the substrate that is not removed and remains intact, to the total area of the adhesive. Typically, a structure is qualified as passing this test when more than 95% of the substrate area covered by the glue is removed upon strain-induced bond failure.
Accordingly, what is needed is a new building construction material and a new method of construction that allows for the maximum reduction of noise transmission at low frequencies, high frequencies, or both simultaneously, while also providing structural integrity. What is needed is a panel designed to optimize both the acoustical and structural performance in typical lightweight frame construction.